This invention relates to integrated circuit memories and more specifically to an integrated circuit memory, especially a dynamic random access memory (DRAM), in which stored contents are sensed from small voltage signals present on bitlines.
Existing DRAMs are generally slower to access than static random access memory (SRAM) or read only memory (ROM). Conventionally, DRAMs have been manufactured as standalone independent integrated circuit chips which store large quantities of data for faster access than magnetic or optical disk media, at relatively low prices and low power consumption. SRAMs and ROMs, by contrast, have usually provided faster access than DRAMs, but often at higher prices and power consumption because these memory types require a larger device count per stored data bit, which adds to cost and density.
Recently, interest has increased in using DRAMs as one of several elements of an integrated circuit, e.g. a xe2x80x9csystem on a chipxe2x80x9d, which may also include logic or linear circuits or other circuit types. Such DRAM elements have been called embedded DRAM or xe2x80x9cEDRAMxe2x80x9d. The goals of using embedded DRAM include obtaining potentially large amounts of easily rewriteable storage with fast access times but at lower cost and power consumption than SRAMs. One problem facing existing DRAMs is that it takes longer to write a memory cell with a new datum than it takes to read or refresh the datum in that memory cell. This problem is understood with reference to FIGS. 1 and 2. Specifically, FIG. 1 shows signals which are active when a prior art DRAM memory cell is being read. A read operation begins with the wordline voltage 10 rising from a quiescent value (in this case, about xe2x88x920.4 volts) to an activated value which makes the access transistor of the memory cell conduct. Charge stored by a capacitor in the memory cell then begins to flow through the transistor on a bitline to a sense amplifier. At the sense amplifier a small voltage difference signal 11 develops between the voltage 12 on a bitline BT and the voltage 14 on a reference bitline BC which is not connected to the memory cell being read. The sense amplifier functions to convert a small swing, e.g. xe2x80x9canalogxe2x80x9d signal between the bitline BT and the reference bitline BC into a full swing logic level signal for storage to or transfer of the datum from the memory cell. After the small voltage signal 11 appears, the sense amplifier is set, i.e. triggered through a signal SETP 16, to amplify the small voltage signal 11 to full swing logic levels. This results in the bitline voltage 12 and the reference bitline voltage 14 separating from their initial small voltage difference to respective predetermined high and predetermined low logic levels, in this case about 1.2 V, and 0.0 V, respectively. The voltage stored in the memory cell is shown in FIG. 1 by curve 18.
By contrast, some write operations in conventional DRAMs take longer to perform than a read operation. With reference to FIG. 2, an operation to write a high logic level, i.e. a xe2x80x9c1xe2x80x9d in a memory cell that currently stores a low logic level, i.e. a xe2x80x9c0xe2x80x9d is known as xe2x80x9cReadxe2x80x940_Modify_Writexe2x80x941.xe2x80x9d This write operation starts by reading the memory cell which contains a xe2x80x9c0xe2x80x9d and then forcing the memory cell to store the opposite value xe2x80x9c1.xe2x80x9d The initial reading step is necessary to prevent the stored contents of memory cells on adjacent bitlines from becoming corrupted. While the one memory cell is being rewritten with a xe2x80x9c1xe2x80x9d from a xe2x80x9c0xe2x80x9d state, memory cells on other bitlines accessed by the same wordline are read and xe2x80x9cwritten backxe2x80x9d with the same data that they already store.
As shown in FIG. 2, the read_modify_write operation begins the same way as the read operation with the wordline voltage 10 rising from the quiescent value to an activated value. Charge stored by a capacitor in the memory cell then begins to flow through the transistor on a bitline to a sense amplifier, where a small voltage difference signal 21 develops between the voltage 22 on a bitline BT and the voltage 20 on a reference bitline BC which is not connected to the memory cell being written. After the small voltage signal 21 appears, the signal SETP 16 sets the sense amplifier, which results in the small voltage signal 21 being amplified into predetermined high and predetermined low logic levels on the reference bitline BC and the bitline BT, respectively, which reflect the original xe2x80x9c0xe2x80x9d value datum stored in the memory cell.
In the prior art DRAM operation shown in FIG. 2, the voltages 22, 20 on the bitline BT and reference bitline BC are forced to new levels only after the sense amplifier is set. After the sense amplifier is set, the voltages 20, 22 advance almost fully towards the high and low logic levels, respectively. Then, the bitline and reference bitline voltages reverse course to reach the opposite levels as required by the write operation. The time required to perform the initial read before writing makes the voltage 24 in the memory cell take longer to rise than in the read operation. In comparison to the read operation shown in FIG. 1, in the read_modify_write operation, the memory cell voltage takes about 30% longer to rise to 90% of final value than it does in the read operation, as evident from comparing the intervals t0-t1 of FIG. 1 with t0xe2x80x2-t1xe2x80x2 of FIG. 2.
Heretofore, the longer time to perform the read_modify_write operation has been considered acceptable. This is because forcing bitline signal levels to new values too soon could potentially corrupt data in other memory cells due to line to line noise coupling between the bitline being written and an adjacent bitline. Heretofore, there has not been a way to quickly write a memory cell with a new value without risking corruption of data in memory cells which are accessed by adjacent bitlines.
Accordingly, among objects of the invention, each of which may operate in the alternative to, or in conjunction with other objects, are the following:
An object of the invention is to perform a write operation to a memory cell in about as little time as a read operation.
Another object of the invention is to quickly perform a write operation to a memory cell without risking corruption of data in memory cells accessed by adjacent bitlines.
Still another object of the invention is to perform a write operation by injecting a small voltage signal onto a bitline and a reference bitline and setting a sense amplifier to amplify the signal into predetermined high and low logic levels for storing into a memory cell.
Accordingly, in an aspect of the present invention, an integrated circuit including a memory is provided which is adapted to write a datum to a memory cell by local bitswitches which apply first and second write voltages having a small voltage difference to a true bitline and a reference bitline prior to a sense amplifier being set, the sense amplifier being adapted to amplify the small voltage difference to a predetermined high voltage and a predetermined low voltage, the true bitline having one of the predetermined high voltage and the predetermined low voltage, which is then written to the memory cell.
In a more preferred aspect of the invention, the memory cell being written and other memory cells are accessed by a wordline. Local bitswitches on such other memory cells not currently being written are adapted to isolate true and reference bitlines coupled to those memory cells prior to the setting of sense amplifiers coupled to those bitlines, such that the stored contents of such memory cells not being written are refreshed at the time that the selected memory cell is written.